Development of positron emission tomography (PET) radiotracers that are specific for pathological tau accumulation in neurodegenerative diseases (tauopathies) represents one of the most active, yet most challenging, areas in neuroscience. Pioneering developments for tau imaging in human subjects have been achieved with [18F]FDDNP, which is effective at imaging hyperphosphorylated tau fibrillar aggregates but does not distinguish between amyloid-β plaques and tau. Development of a selective and specific imaging agent is important for advancing our understanding of tauopathies, improving differential diagnostic accuracy, accelerating drug discovery and monitoring of therapeutics.